245E-257-Raw Asteroid

Most asteroids that we observe have been baked by the Sun for billions of years. As a result, any ices or frozen gases that they may have once possessed are long gone. Comets on the other hand have spent so much time far from the Sun that they still contain volatile materials. As a typical comet approaches the Sun, the frozen gases and ices it contains evaporate to form a beautiful coma and a long tail.